You married mine…

“Daddy,” said a six-year-old boy, “I’d like to get married.”
“Sure son,” said his father. “Anyone special in mind ?”
“Yes,” answered the boy, “Grandma.”
“Now, wait a minute,” said his father.
“You don’t think I’d let you marry my mother, do you ?”
“Why not ?” the boy asked, “You married mine..”